The present invention relates to a process for mixing polymerization raw materials by continuously mixing trioxane, a comonomer which is copolymerizable therewith and a polymerization catalyst and feeding them into a polymerization reactor in the production of polyacetals, and an apparatus therefore.
Oxymethylene copolymers containing about 60 to 99.6% of oxymethylene units and a balance of repetition units of (--c--c-- ) have been known as a very useful plastic for industrial uses.
Processes for producing the above-mentioned oxymethylene polymers have heretofore been studied, and above all, processes of employing trioxane as a starting raw material have been particularly developed due to easy purification of trioxane.
Processes for producing oxymethylene copolymers employing trioxane include a batch system and a continuous system.
In the batch production process, liquid trioxane, a comonomer and a polymerization catalyst, each in a fixed amount, are fed into a reactor and mixed together with stirring to obtain polymer. This process, however, has drawbacks that the reaction product forms a mass with the progress of a rapid polymerization reaction, adequate temperature control of polymerization substances is difficult, milling of final polymer is not easy and discharge of product is difficult.
On the other hand, in the continuous process, trioxane and a comonomer ring are subjected to bulk polymerization while the reaction temperature is adjusted, and oxymethylene copolymer is obtained as a milled product. In an example, a reaction mixture is fed into a continuous polymerization apparatus from its inlet, and polymerized in a reaction zone. The resulting polymer is mechanically milled, carried and taken out of its exit. According to such a continuous bulk polymerization, the amounts of solvent and catalyst employed are small and a copolymer can be obtained at a high conversion, and hence the process is a very advantageous one as a commercial process for polyacetal resins.
However, for completing this technique, certain problems should be solved. One of them is to develop a mixing method for enabling to continuously mix together trioxanes, a comonomer and a polymerization catalyst and quantitatively feed them into a continuous polymerization reactor. When trioxane containing substantially no solvent is mixed with a polymerization catalyst, polymerization reaction immediately starts and the reaction mixture solidifies. Thus, polymer adheres to the tip end of the catalyst-feeding nozzle, and feed often becomes impossible, resulting in an obstacle to a smooth continuous polymerization. In order to prevent this, processes such as incessant washing of the tip end of the catalyst-feeding nozzle with a large amount of a solvent, or adding large amount of a solvent to trioxane and comonomer to be fed, have been proposed, but these processes are deviated from the object of the bulk polymerization and commercially unadvantageous. Treatments such that the tip end of the catalyst-feeding nozzle is often taken out and cleaned may be carried out, but, in these cases, such problems are raised that mixing-in of moisture in air or variability of the amount of catalyst fed occurs, resulting in constant molecular weight of polymer extruded from a continuous polymerization reactor as well as inconstant conversion.